1) Field of the Invention
The present invention relates to a radar apparatus designed to process a beat signal forming a mixture of transmitted and received signals on a radar wave for detecting a target which has reflected the radar wave.
2) Description of the Related Art
So far, an FMCW type radar using a millimetric-wave band (which will be referred to hereinafter as an “FMCW radar”) has been known as one of vehicle-mounted radars applicable to, for example, an inter-vehicle distance control apparatus made to detect a vehicle (preceding vehicle) existing in a forward direction for maintaining the inter-vehicle distance properly with respect thereto or an inter-vehicle distance alarm apparatus made to make a notification to the driver when the inter-vehicle distance becomes shorter than a predetermined distance.
This FMCW radar uses a radar wave modulated such that the frequency increases and decreases (rises and falls) linearly with the passage of time to form a triangular waveform, and acquires information about a radar wave reflecting target on the basis of a beat signal obtained by mixing the transmitted wave on the radar wave and the received signal on the radar wave (reflected wave) from the target.
Concretely, with respect to a rising section (zone) in which the radar wave frequency increases and a falling section in which it decreases, the frequency analysis processing, represented by the fast Fourier transform, is conducted on the beat signal, thereby obtaining a power spectrum in each section of the beat signal. Moreover, the peak frequency components extracted from the power spectrums are properly combined in the both sections and the frequency of the combined peak frequency components (which will be referred to hereinafter as a “peak pair”) is applied to a well-known equation in the FMCW radar to obtain a distance to a target specified by the peak air or a relative speed thereto.
Meanwhile, in the FMCW radar, as shown in FIGS. 9A to 9C, a noise occurs in a low-frequency region of a beat signal due to the reflection at a short distance from a radome or the like mounted at an opening portion for the output/input of a radar wave, the coupling between transmission/reception antennas, or the like.
For this reason, if a peak develops in a region NA in which the low-frequency noise occurs (which will be referred to hereinafter as a “low-frequency noise region”), difficulty is experienced in extract this peak (see FIG. 9B), which makes it difficult to detect a target which has produced the peak.
As the countermeasures against this, there has been proposed an apparatus designed such that the switching of a transmitted signal or received signal is made at approximately several MHz to generate a frequency component of a beat signal in an intermediate frequency band non-susceptible to the influence of the low-frequency noise for, in this state, converting the beat signal into a proper frequency after removing the low-frequency noise (for example, see Japanese Patent Laid-Open No. HEI 5-40169.
However, in this apparatus, in addition to the employment of the configuration of the FMCW radar, there is a need to use an oscillator, mixer, filter and other devices for production and processing of a signal in an intermediate frequency band, which leads to the complication and size-expansion of the apparatus and, hence, an increase in manufacturing cost.
In addition, in the case of tracking a preceding vehicle through the use of the FMCW radar, if the tracked preceding vehicle gets between roadside things or travels side by side with another preceding vehicle, on the power spectrum, the peak stemming from the tracked preceding vehicle falls into obscurity due to the peaks from the roadside things or the other preceding vehicles, as in the case of the low-frequency noise, difficulty is encountered in detecting this peak from the tracked preceding vehicle.
Add to it that, because the peaks from the roadside things or the other preceding vehicles do not always develop in a constant frequency band unlike the flow-frequency noise, the apparatus disclosed in the aforesaid document cannot cope with such a situation.